Master Thesis

Analyse der mittelfristigen Populationsentwicklung von Cordulegaster bidentata und Cordulegaster boltonii an Waldquellen in der Hersbrucker Alb anhand von Larvennachweisen

Katrin Mayer (11/2022-05/2022)

Support: Carl Beierkuhnlein, Kai Frobel

The study focused on evaluating the medium-term population development of Cordulegaster bidentata and Cordulegaster boltonii on larval records. The basis for this was provided by recordings on 173 forest spring streams in the Hersbrucker Alb area near Nuremberg. In this area, data on the occurrence of the two Cordulegaster species have been collected at irregular intervals since 2006 in 476 forest spring streams. In addition, corresponding repeat sampling was carried out from 2020 up to and including February 2022. This comparative study showed above all the extinction of very small occurrences of C. bidentata at source streams with falling discharge. This loss of population supports the hypothesis that the larvae population of C. bidentata, that specializes in forest springs, is decreasing in the study area. In just as many headstreams, the existing C. bidentata locations were recolonized by C. boltonii, which supports the hypothesis that the larvae population of C. boltonii is increasing in the study area. Most of these springs currently populated by both species, did not show any decrease in the amount of discharge. This and the amount of fine sediment suitable for burrowing might be of crucial importance for the development of high larvae densities. In general, there seems to be a certain dynamic in the development of Cordulegaster occurrences, which is characterized by decreases and increases in larval density, but also by stable populations. A displacement of the larvae of C. bidentata by C. boltonii larvae entering the habitat could not be determined, contrary to the assumed hypothesis. Rather, both species occur together over large ranges at the source stream and evidence of the larvae of C. boltonii is only missing in the uppermost areas at the source of the source, while C. bidentata larvae also colonize these sections. Therefore, in the study area, a coexistence of both species instead of displacement is currently indicated. It remains to be seen whether this association with the expected negative climate-related changes in precipitation and thus decreases in spring discharge will persist.